1. Field of the Invention
The present invention relates to an exposure apparatus (scanning exposure apparatus) for exposing a substrate to light via a reticle and a projection optical system while the reticle and the substrate are moved in a scanning direction.
2. Description of the Related Art
In a scanning exposure apparatus that exposes a substrate such as a semiconductor wafer, a focus sensor is configured to only measure discrete points on the wafer surface. Therefore, in the case where there is an uneven pattern in the base of a resist, from a viewpoint of focus precision, it is preferable to measure the offset value related to the unevenness of the base pattern in advance. Japanese Patent Laid-Open No. 09-045608 proposes a method in which a wafer is exposed as the height of the wafer is adjusted based on the value obtained by subtracting the offset value from the focus measurement value (the height of the wafer surface). In Japanese Patent Laid-Open No. 09-045608, the offset value is obtained based on the measurement values of the height of the wafer surface in a plurality of sample shots.
However, if there is a singular region, for example, a region having an abnormal inclination on the wafer surface, the offset value varies depending on how the sample shots are selected. Description will be made with reference to FIG. 2. Patterns 202A to 202C are formed on a wafer substrate 201. In order to obtain an offset value corresponding to the pattern structure, the height distributions of the sample shots 202A to 202C are measured. The measurement results are simply averaged as shown by 203A to calculate an averaged height distribution (corresponding to the offset value distribution) 203B. In this example, the wafer substrate is nearly second-order-paraboloidal, and the shot 202A and the shot 202C are disposed substantially symmetrically with respect to the shot 202B. Therefore, by simply averaging the measurement results, an offset value can be obtained with comparatively high precision.
However, if dust enters between the wafer substrate and the wafer substrate stage or the wafer substrate warps in the circuit forming process, the substrate is not always second-order-paraboloidal. The situation will be described with reference to FIG. 3. In FIG. 3, since the wafer substrate 301 warps due to the processing in the pattern forming process, the wafer substrate is not paraboloidal. If the height distributions of patterns 302A to 302C formed on the substrate 301 are simply averaged as shown by 303A, the averaged height distribution inclines, for example, as shown by 303B due to the shape of the substrate. That is to say, an offset value distribution corresponding to the pattern structure cannot be calculated with high precision. As a result, the height measurement value of the substrate surface is corrected using such an offset value, and therefore the focus precision is deteriorated.